The present invention relates to a method and apparatus for treating lime mud in order to regenerate the lime mud, which has been separated in connection with causticizing process and is mainly CaCO.sub.3, to form CaO in a rotary kiln.
By prior art methods, lime mud is introduced at a dry solids content of 60 through 70% in the top end of a rotary kiln wherefrom it slowly glides downward through drying, heating and reaction zones. The drying zone, in which heat is transferred from flue gases to the lime mud mainly through convection, also includes chains employed to improve the heat transfer. The chains contact the lime mud when the kiln rotates. In order to obtain good results, the heat treatment of the lime mud in the kiln must take place slowly, requiring utilization of a long kiln, and taking up a large space.
It has been possible to shorten a lime reburning kiln by carrying out the drying and the preheating of lime mud in pneumatic driers with flue gas prior to introducing the lime mud in a lime reburning kiln.
As the wet lime mud obtained from the filter is difficult to handle, dried lime mud has been mixed into it before it has been fed to a pneumatic drier. The equipment needed for the pretreatment of lime mud has been complex and it has required a complicated control and adjustment system.
The object of the invention is to provide a simple method and an apparatus for drying and preheating lime mud.
The invention is based on the observation that if lime mud is thickened in a lime mud filter to a dry content of over 75%, the lime mud can be introduced directly to a pneumatic drier and no hammer mills or mixer screws are needed to disrupt the lumps. The lime mud is then so dry that the drier will remain open.
The method of the invention is mainly characterized in that the lime mud is thickened in a lime mud filter to a dry solids content of over about 75%, the lime mud from the filter is dried and preheated in a pneumatic drier by means of flue gas from a lime reburning kiln, and the dried lime mud is separated from the flue gases and introduced in the lime reburning kiln.
The apparatus of the invention is characterized in that it comprises a drying conduit for guiding flue gas from the lime reburning kiln to a cyclone, a feed screw for dosing the lime mud from the lime mud filter to the vertical portion of the drying conduit and means for transferring the dried lime mud separated in the cyclone to the lime reburning kiln.
The method and the apparatus of the invention provides at least the following advantages over conventional lime reburning kilns: The lime reburning kiln is approximately 30% shorter than conventional kilns, which dispense with at least one support. The investment in the kiln is smaller. Space is saved (which is important in many cases). The end temperature of the flue gases decreases, which reduces the oil consumption of the lime reburning kiln. The heat transfer in the drying conduit and in the cyclone is much more efficient than in the conventional chain zones; and the chain zone and the lifting zone can be shorter than in conventional kilns.
Compared with the prior art lime reburning kilns provided with pneumatic driers, the apparatus of the present invention is less expensive and simpler and consumes remarkably less energy.